The principal thrust of the project during the past year has been to determine mechanisms of transcapillary exchange of ions and molecules in bone. To validate past studies utilizing indicator dilution techniques, it has been necessary to determine the ratio of I.F.S. (interstitial fluid space) of bone to the vascular space of bone. Furthermore, microvascular studies indicate that cortex and marrow circulation are mainly in parallel. Altering the method of reinforcing plate fixation of experimental tibial fractures with methyl methacrylate indicate that this can be accomplished without decreasing fracture site blood flow. Preliminary studies have been accomplished that make it feasible to measure antibiotic content of cortical and cancellous bone by 14C-labeled penicillin. Combined with indicator-dilution techniques to measure permeability ratios of penicillin and 85Sr, it should be possible to study pharmacokinetics of antibiotics in bone. BIBLIOGRAPHIC REFERENCES: Sorensen, W.G. and Kelly, P.J.: Effects on canine tibial blood flow and fracture healing by acrylic cement-augmented stabilization. Trans. 22nd Annual Meet. Orth. Res. Soc. 1:129, 1976, abstract. Lopez-Curto, J.A., Hughes, S.P.F., Kelly, P.J. and Bassingthwaighte, J.B.: Interstitial fluid space in bone. Fed. Proc. 35:850, 1976, abstract.